A mail stacker is usually a part of a mailing machine, addressing equipment or mail creation equipment. Mail stackers can be classified into two types: top stackers and bottom stackers. In a top stacker, a later mail piece is stacked on top of the earlier ones. In a bottom stacker, a later mail piece is placed at the bottom part of the stack. In some applications such as addressing and inserting systems, mail pieces are required to be stacked in a certain serial order. For example, mail pieces are required to be stacked in a forward serial order in order to be eligible for a postal discount. The addressing information is typically printed on top of the mail pieces.
For mail pieces printed in a 1-to-N order, the topmost mail piece in a mail stack having a forward serial order is always printed earlier than the rest of the stack. However, top stackers will reverse the order of the mail pieces to an N-to-1 order while stacking. In order to keep a forward serial order when using a top stacker, an application software can be used to reverse the serial order when addressing. The use of order-reversing software adds considerable complexity to the mail processing system, especially for jam recovery.
Thus, in a mail system requiring a forward serial order, it is advantageous and desirable to use a bottom stacker to reverse the serial order while stacking.
Bottom stackers are known in the art. For example, Keane et al. (U.S. Pat. No. 6,398,204 B1) discloses a belt stacker wherein mail pieces are separately fed by an edge feeder to a stacking deck on the edge of the stacker at the upstream end of the stacking deck. The mail pieces already in the stack are moved by a conveyer belt toward the downstream, away from the edge feeder. At the same time, a stack support is used to keep the stacked mail pieces in an upright position while they are moved downstream. The stack support must be moved toward the downstream end to allow additional room for the stack to grow. In Keane et al., the stack support is either manually relocated or moved by the conveyor belt. Marsullo et al. (U.S. Pat. No. 5,709,525) also discloses a bottom stacker, wherein a pusher mechanism is used for sealing the incoming envelope and pushing the sealed envelope onto a horizontal deck for stacking. In order to keep the stacked envelope in an upright position, a stack support is placed on top of the stack. The stack support is urged by a spring disposed on the back side of the stack support against a rear wall of the stacking deck.
This type of bottom stacker requires a large footprint in that the size of the stacker is determined mainly by the size of the stack, and not the size of the mail pieces in the stack.
It should be noted that during normal operation of a bottom stacker, an operator may unload small portions of the stack from the top of the stack as mail continues to be added to the bottom of the stack. This is typically done one handful of stacked mail at a time. One handful may be a portion of the stack between one and five inches high. Stacker unloading may constitute the operator removing small segments of a larger stack from the top of the stack multiple times and placing each small segment of the stack in a storage container. Typically, the stacker can continue to operate while this unloading operation is underway.
For high speed stacking systems, the option of unloading the small portions from the top of the stack may require continual operator attention. Alternately, the operator can choose to wait until a larger stack has accumulated in the stacker and unload larger and heavier stacks from the stacker less frequently. If the operator chooses to unload the stacker less frequently, but in larger batches, it will save the operator substantial amount of time, thus freeing the operator to attend to other tasks while the system continues to operate. However, if the stacker unloading operation requires that the operator shut down the stacking system while unloading it, then both the operator and the system will be less efficient. It will be desirable to unload large stacks without having to shut down the system.
It is, however, commonly known by mail equipment operators that picking up and handling large stacks of mail pieces is often an awkward and unwieldy process, requiring a high degree of operator skill and experience. Large vertical stacks of mail pieces often have an undesirable aspect ratio of height to cross sectional area, and the stack tends to be quite unstable when it is not supported on an edge. This stack instability occurs when an operator attempts to pick up and handle a large, unsupported segment of a vertical stack of mail. When an inexperienced operator attempts to pick up a larger stack of mail, it is quite common for the stack to collapse, or portions of the stack to fall away from other portions of the stack, and thus the mail pieces are no longer in the correct order. The operator must take time to restore the stack back into the correct order. Because of the frequency of such stack collapse problems, mail workers find it desirable to handle mail in trays. For the USPS, a typical mail tray will handle a stack of mail that is approximately 24 inches long. It is thus advantageous and desirable to provide a method and device to assist unloading of stacked mail without the risk of collapsing the stack during the unloading operation. It is further advantageous to unload up to approximately 24″ stacks of mail directly into a standard mail handling container such as the mail trays used by the USPS as part of their standard mail handling processes.